(First Technique and Its Problems)
In order to deposit polysilicon on the surface of, e.g., an intermediate semiconductor device at an intermediate stage of manufacturing processes by means of the reduced pressure CVD method, it is necessary to chemically clean, prior to such deposition, the surface of a polysilicon layer on, or an impurity diffused layer in, the substrate. Thereafter, the substrate is washed or placed in atmosphere so that a very thin oxide film called a natural oxide film grows on the substrate surface. The oxide film grows further when the substrate is introduced into a reaction tube for deposition of a polysilicon film. Under such conditions, if a polysilicon film is deposited on the oxide film by the CVD method, a failure of the electric connection will occur. Such a problem is particularly serious where the underlying layer is a polysilicon layer. In order to avoid such failures, a so-called ion mixing method has been adopted whereby after the deposition of a polysilicon film on the substrate by the CVD method, ions are implanted to destruct the oxide film for achieving an electrical connection. This ion mixing method necessitates using an expensive ion implanting apparatus, and, in addition, the semiconductor device is subject to ion radiation damage.
There is a method where a silicon nitride film is deposited on a polysilicon film formed by the CVD method to use the former as a mask for field oxidation. Also in this method, an oxide film will grow on the polysilicon film. The presence of an oxide film between the polysilicon film and silicon nitride results in abnormal oxidation in many cases.